The present invention relates to an electric fuel pump comprising a casing in which is disposed a stator armature of an electric motor, a rotatable shaft to which is fixed a rotor winding of the motor and which is torsionally connected to a pump impeller rotor, and a pair of covers for closing the ends of the casing and defining between them a chamber within the casing for the fuel.
Pumps of this type are known in which to the said first cover is rigidly connected a fixed shaft which constitutes the support for the rotor winding and for the other parts of the pump which are connected to this latter. In this constructional arrangement the rotor winding is formed on a tubular element which is traversed by the fixed shaft and which is provided with a pair of end bearings supported on the outer surface of the shaft itself. On a first end of the rotor winding, which faces the impeller rotor, are formed teeth which couple with corresponding teeth of the impeller rotor itself for the purpose of driving it to rotate.
In order to fix the rotor winding in the correct axial position with respect to the impeller rotor there is normally used a resilient ring which is inserted in a corresponding annular cavity of the fixed shaft and which constitutes a shoulder for the second end of the rotor winding.
The pump impeller rotor, which is also directly rotatable on the fixed shaft, is housed within a casing which is formed by the first cover, a plate fixed to this and a spacer of annular form interposed between the first and the second; the impeller rotor is therefore rotatable between a pair of substantially parallel walls of the cover and the plate, and normally has a plurality of radial cavities in which move rollers cooperating with suitable guide surfaces formed on the interior of the said spacer: with this constructional arrangement there is formed a volumetric pump which is able to cause fluid to circulate from a suction hole formed in the first cover to a discharge hole formed within the plate and axially along the chamber which is defined within the casing and by the two pump covers.
Pumps of the type briefly described have various disadvantages.
First of all, the first-defined lateral surfaces of the first cover and the plate which define the cavity in which the pump impeller rotor is rotatable and the corresponding lateral surfaces of the impeller rotor itself can be imperfectly parallel and therefore give rise to non-uniform clearances which cause losses by escape of the fuel and therefore are sources of a considerable reduction in the volumetric efficiency of the pump itself. This disadvantage is due to the fact that the fixed shaft, which is connected to the first cover, can be imperfectly orthogonal to the inner surface of the cover itself and therefore give rise to the said errors in parallelism between the lateral surfaces of the impeller rotor and the interior of the cover and the plate, the impeller rotor itself being directly rotatable on the fixed shaft. This disadvantage can therefore easily occur since the connection of the fixed shaft to the first cover is achieved by fitting the end of the shaft itself into a corresponding hole of the cover using a press: it is therefore evident that during this operation errors of orthogonality between the axis of the shaft and the cover, or small deformations of the shaft itself, can easily occur. The same constructional and assembly errors of the unit constituted by the first cover and the fixed shaft can be the source of another disadvantage constituted by the inaccurate centring of the rotor winding with respect to the stator armature or in the production of abnormal stresses between the bearings of the first and the fixed shaft.
Moreover, in pumps of the type described there are always not inconsiderable axial clearances between the rotor winding and the pump impeller rotor which tend to increase in use: this disadvantage arises from the manner in which the rotor winding is axially fixed with respect to the fixed shaft by means of the said resilient end ring, which constitutes a shoulder for the winding itself.
Moreover, the said axial clearances can depend, in an important measure, on the dimensional tolerances of the various rotary members which are supported by the fixed shaft and which are therefore axially fixed by the said resilient ring: it is evident that for the purpose of maintaining this axial clearance within reasonable limits the various parts mentioned above must be worked with very strict tolerances.
Further, the structure of the pump of the type described is very complex because of the numerous parts of which it is constituted: in particular the rotor winding is provided with an internal tubular element for supporting it and for housing the bearings provided for supporting the winding itself in its rotation with respect to the fixed shaft.
The operations necessary for balancing the rotor winding before its assembly on the pump are very difficult since, in order to support it for rotation during these operations it is necessary to introduce a mandrel into the interior of the tubular element and the first-mentioned bearings, the mandrel having a suitable form and dimensions and the ends of which can be engaged on suitable seats of the balancing machine.
Finally, the various operations which precede the assembly of the pump, such as those connected with the storing and movement of the various parts and the various units of the pump, are rather difficult because of the considerable size of the assembly constituted by the first cover and the fixed shaft connected to it: this assembly thus has an axial length depending on that of the fixed shaft which is substantially equal to that of the finished pump, and a diameter corresponding to that of the first cover which is substantially equal to the diameter of the casing of the pump.